Magnetic shielding for electrical transformer



Nov. 3, 19 70 c. J. BELL ETAL MAGNETIC SHIELDING FOR ELECTRICAL TRANSFORMER Filed April 9. 1969 4 Sheets-Sheet l PRIRA FIG.|.

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lNVENTORS Clifford J. Bell and Harold R. Moore BY WITNESSES Jamil f 2M ATTORNE NOV. 3, 1970 c, J, L ETAL 3,538,472

MAGNETIC SHIELDING FOR ELECTRICAL TRAN. -SFIORMER Filed April 9. 1969 4 Sheets-Sheet 2 FIG.3.

Nov. 3, 1970 c. J. BELL ETAL 3,538,472

v I MAGNETIC SHIELDING FOR ELECTRICAL TRANSFORMER Filed April 9. 1969 .4 Sheets-Sheet :5 v

Nov. 3,191o CHLBELL ETAL 3,538,472

- MAGNETIC SHIELDING FOR ELECTRICAL TRANSFORMER.

Filed April 9. 1969 4 Sheets-Sheet 4.

United States Patent 01 3,538,472 Patented Nov. 3, 1970 hot:

3,538,472 MAGNETIC SHIELDING FOR ELECTRICAL TRANSFORMER Clifford J. Bell and Harold R. Moore, Muncie, Ind., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 9, 1969, Ser. No. 814,633 Int. Cl. H01f 15/04 US. Cl. 336-84 9 Claims ABSTRACT OF THE DISCLOSURE An electrical transformer of the shell-form type in which the support members for maintaining the coils in assembled relation with the magnetic core are formed of a plurality of bundles of stacked magnetic laminations, which also perform the function of providing low reluctance magnetic circuits for the leakage flux of the transformer.

CROSS REFERENCE TO RELATED APPLICATION Certain of the apparatus disclosed but not claimed in this application, is disclosed and claimed in copending application Ser. No. 814,637, filed Apr. 9, 1969, in the name of Clifford I. Bell, which application is assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION Field of the invention The invention relates in general to electrical transformers, and more specifically to new and improved coil support and shielding means for transformers of the shellform type.

Description of the prior art In the prior art, it is common to direct stray or leakage flux inside a power transformer away from the tank walls, and end frames, to reduce losses and prevent overheating by magnetic shunts formed of stacked laminations of grain oriented silicon steel. The stacks of laminations are generally held in assembled relation by welding.

In electrical transformers of the shell-form type, a plurality of electrical coils are assembled about a leg portion of the magnetic core. Metallic support beams or members and wedges are inserted through the openings in the pancake coils at the top and bottom of the stack of core laminations, between the magnetic core and the end portions of the coil openings, to support the coils and prevent their movement due to short circuit stresses, to clamp the laminations tightly together in the tongue portion of the core which extends through the coil openings, and to support the pancake coils in the proper assembled relation with the core structure. The support beam is usually constructed of a nonmagnetic material, even though it is considerably more costly than magnetic materials, in order to reduce the losses and heating of the beam due to the leakage flux from the pancake coils, which is particularly high at the location of the support beam and coil wedges through the openings in the coils. However, even the use of nonmagnetic materials, such as stainless steel, will not prevent excessive heating of the coil sup port beam, making it necessary to weld bundles of magnetic laminations to the support beam and core wedges, to direct the leakage flux into these low reluctance paths. US. Pat. No. 2,370,045 discloses prior art support and shielding structures of this type for shell-form transformers.

While the shielding of the coil support beam and wedges with magnetic material reduces the heating of these components, heating due to leakage flux, in the coil support beam, wedges, tank wall and end frames, is still a problem, which increases in severity as the rating and size of the transformer is increased, due to the increased density of the leakage flux, and the increased size of the metallic parts required to provide the necessary increase in physical strength. The welding of the bundles of laminations raises their losses, and the welding of the bundles to the metallic coil support beams and wedges necessarily provides a space between adjacent bundles in order to weld them to the support beam. This space reduces the efiiciency of the magnetic shield. Further, the bundles of laminations which shield the coil support beam, tank and end frames, form magnetic circuits by butting the end-s of the bundles together. Tolerances in the tank wall and end frames are such that the ends of the adjacent bundles usually have large gaps between them, which increases the reluctance and losses in these magnetic circuits.

SUMMARY OF THE INVENTION Briefly, the present invention overcomes the disadvantages of prior art shielding structures for shell-form power transformers, by providing new and improved magnetic shielding structures which eliminate welding, eliminate the coil support beams, and eliminate gaps between adjacent bundles at the corners and ends of the bundles. Specifically, the bundles of laminations are held in assembled relation by a tightly wound fabric tape, which is impregnated with a resinous adhesive system, which bonds the laminations and tape into a solid beam-like structure. The ends of the laminations in a bundle are offset to provide tongue-and-groove joints for joining the ends of adjacent bundles together, which allows a substantial tolerance in the construction of the bundles and in the components they are to shield, without substantially increasing the reluctance of the joint.

The new and improved bundles of bonded laminations are strong enough to form the coil support beams required at the ends of the coil openings adjacent the magnetic core leg portion. Thus, the bundles of laminations provide the mechanical function of supporting the pancake coils, and the magnetic function of providing a highly eflicient low reluctance path for the leakage flux. Since the bundles of laminations are not Welded to a backup plate or beam, they may be placed in contacting side-by-side relation to substantially fill the coil opening adjacent the ends of the magnetic core structure.

BRIEF DESCRIPTION OF THE DRAWING Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is a fragmentary elevational View, in section, of a transformer constructed according to the teachings of the prior art;

FIG. 1A is an enlarged fragmentary view of a portion of the transformer shown in FIG. 1;

FIG. 2 is an enlarged fragmentary view illustrating how the transformer of FIG. 1 may be modified to provide support for the core laminations, and shielding, adjacent the curved corners of the coil openings;

FIG. 3 is an enlarged fragmentary view illustrating another embodiment of the modification of the transformer shown in FIG. 2;

FIG. 4 is a perspective view, partially cut away, of a shell-form transformer having a magnetic shielding arrangement constructed according to the teachings of the invention; and

FIG. 5 is a perspective view of magnetic shielding bundles constructed according to the teachings of the invention.

3 DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the drawings, and FIG. 1 in particular, there is shown an elevational view, in section, of electrical inductive apparatus 10, such as a transformer or reactor, which may utilize the teachings of the invention. Specifically, inductive apparatus includes a magnetic corewinding assembly 12 disposed within a tank 14, which may be filled with a suitable insulating and cooling dieelectric, such as oil. The electrical bushings for bringing electrical conductors through the wall of the tank 14 for connection to the windings therein, are not shown in order to simplify the drawing.

The magnetic core-winding assembly 12 is of the shell-form construction, having a plurality of pancake coils, such as pancake coil 16, disposed in inductive relation with a leg portion 17 of the magnetic core structure 18. Magnetic core structure 18 has first and second similar assemblies or sections 20 and 22. Each of the magnetic core assemblies 20 and 22 are formed of a plurality of metallic, magnetic laminations 24, such as grain oriented silicon steel, which are stacked to form a rectangular structure having at least four interconnected stack sections arranged to define at least one window or opening through which the coil turns may pass. The two assemblies 20 and 22 are disposed in side-by-side relation, with their adjacent stacked sections forming the winding leg portion 17, about which the pancake coils are assembled. Thus, in the customary construction of shell-form power transformers, the pancake coils extend outwardly from the top and bottom surfaces of the magnetic core assembly 18.

The pancake coils, such as pancake coil 16, are each constructed of a plurality of turns 26 of electrical conductor, such as copper or aluminum, with the turns being insulated and wound to provide a substantially rectangular configuration having rounded outer corners, and defining an opening 19 through which the winding leg portion 17 of magnetic core structure 18 may pass. The opening 19 in each of the pancake coils is also substantially rectangular in shape, having substantially straight sides joined by curved corners 28, 30, 32 and 34. The pancake coils are disposed in side-by-side relation, with their openings in alignment, and they are interconnected to form the Winding, or windings, of the electrical inductive apparatus or transformer 10. Transformer 10 may be single or polyphase, and it may be of the isolated winding, or of the autotransformer type.

In order to support the pancake coils, such as pancake coil 16, against movement in a direction parallel with the major surfaces of the pancake coils, such as may tend to occur during short circuit conditions due to differences in the locations of the electrical centers of the pancake coils, which produces a force component parallel with the major surfaces, in order to support the pancake coils and insulating barriers, such as insulating barrier 36, and in order to clamp the laminations 24 tightly together in the tongue or leg portion 17 of the magnetic core assembly 18, support means is disposed to extend through the coils, between the coils and leg portion 17, at the upper and lower surfaces defined by the openings in the pancake coils and upper and lower surfaces of leg portion 17. For example, as shown in FIG. 1, metallic T-beams 38 and 40, along their magnetic shielding members illustrated generally as a plurality of bundles 42 and 44 are provided at the upper and lower spaces, respectively. The bundles 42 and 44 are formed of a plurality of stacked metallic laminations which are welded together, and which are then welded to the T-beams with the major surfaces of the laminations being perpendicular to the T-beams.

The metallic T-beams 38 and extend through the coils and are supported at their extreme ends outside the coils. Wedges, as required, are forced between the T-beams and pancake coils to compress the laminations of the leg portion 17 tightly between the upper and lower coil support members. The pancake coils and insulating barriers are also pressed together in a direction perpendicular to their major surfaces, between end frame structures attached to the tank walls, to hold the pancake coils against movement due to the component of force generated during short circuit conditions which tends to separate the coils in a direction normal to their faces.

The leakage flux from the pancake coils has a high density adjacent the coil support T-beams 38 and 40, with the flux density being especially high adjacent the rounded corners of the pancake coil openings. In order to illustrate the problem of shielding adjacent the rounded corners of the coil openings, the portion of the transformer 10 shown in FIG. 1 which is within the circle A, is shown enlarged in FIG. 1A As illustrated in FIG. 1A the leakage flux, illustrated by lines 46, intersects the rounded corner of the coil opening substantially perpendicular to the curved surface. The leakage flux 46 causes overheating of the T-beam 40 at its lateral edges, as it is difficult to extend the bundles 44 of metallic laminations to the edges of the T-beam due to the curvature of the coil opening. Further, the edges of leg portion 17 and the edges of the bundle 44 of shielding laminations which are adjacent corner 32 overheat, as the leakage fiux in this area does not strike the thin edges of these laminations perpendicular thereto. Therefore, eddy currents are created in the end laminations of the shielding bundles and the end laminations of the leg portion 17, which cause excessive heating and losses in these areas.

The overheating of the metal disposed adjacent the curved corners of the coil opening may be substantially reduced by providing specially shaped magnetic shunt members which closely conform to the curved coil openings. The specially shaped magnetic shunt members provide a low reluctance shunt path adjacent the edges of the T-beam, its magnetic shielding members, and magnetic core, to prevent the leakage flux from entering these members and causing heating and losses due to eddy currents.

FIG. 2 is a fragmentary view illustrating how the transformer 10 shown in FIG. 1 could be modified to reduce the heating adjacent the curved coil corners, with the fragmentary view shown in FIG. 2 being the same area encompassed by the circle A of FIG. 1. Like reference numerals in FIGS. 1 and 2 indicate like components. More specifically, FIG. 2 illustrates a special magnetic shielding member 50, which is formed of a cast resinous insulating material 51, such as an epoxy resin system, which is filled with magnetic material 52. The T-beam 40 shown in FIG. 1, is given the reference numeral 40', as it has a width which extends only to the point where the coil opening starts to curve at its corners. The solid magnetic shielding member 50 now provides the support required for the laminations 24 of the magnetic core leg portion 17, which was heretofore provided by the T-beam, and it additionally provides support for the laminations where they are unsupported by prior art structures. By reducing the width of the T-beam 40, it will extend approximately to the same location as the outer edge of shielding bundle 44, which allows the magnetic shielding member 50 to be constructed with a fiat surface 53 which is disposed against the T-beam 40 and shielding bundle 44, a flat surface 55 which is perpendicular to the first flat surface 53, which is disposed against the bottom lamination of the leg portion 17, and a curved surface 57 which closely conforms to the curved surface 30 of the coil opening 19.

The resin system of which the magnetic shielding member 50 is formed may be any suitable resin which will maintain its strength at the elevated temperatures to which it will be subjected during normal usage in the transformer, and it should also be resistant to attack from the cooling fluid utilized in the transformer. In general, the resin should be a thermosetting resin, with the epoxy resins being excellent because of their excellent physical strength at ambient and elevated temperatures, and their resistance to attack from transformer oils and other coolants utilized in transformers. Since suitable epoxy resin systems are well known in the art, it is not necessary to describe a special epoxy resin system in detail.

Magnetic shielding member 50 is filled with metalmagnetic particles 52, such as metallic shot and/or metallic powders. The metallic shot and powder should, in general, be selected to provide the maximum concentration of filler in the magnetic shunt member 50, in order to provide a low reluctance magnetic circuit. The filler particles may be a uniform size, or they may be graded, to provide a predetermined range of sizes.

Suitable magnetic shunt members have been cast of epoxy resin filled with 200 mesh powdered iron, utilizing the ratio of to parts of resin to 80 to 90 parts of the powdered iron, by weight. They were formed in a mold which had sufiicient length to allow the resulting cast magnetic shunt to be disposed through the entire length of the window of an electrical phase, and to intercept or butt against magnetic wall shields disposed on the tank at either end. The mold shape closely conformed to the configuration of the opening into which the magnetic shunts were to be inserted, providing additional support for the tongue iron. Magnetic shield members constructed in this manner have been very effective in preventing heating of the steel parts separated from the flux source by the magnetic shield members, and the shunts themselves experienced very little heating, which illustrates that they form a low reluctance magnetic shunt circuit.

FIG. 3 is a fragmentary view which illustrates still another embodiment of the cast type corner shield. FIG. 3 encompasses the same general area as the fragmentary view of transformer 1 shown in FIG. 2, and illustrates how the transformer of FIG. 1 would be modified according to this embodiment. In this embodiment, instead of filling the specially formed shunt member of discrete metallic particles, such as metallic shot and/ or powdered iron, the efiiciency of the shunt is increased by embedding oriented bundles of metallic laminations within the shield member. More specifically, as shown in FIG. 3, a specially formed magnetic shunt member 60 is cast in a special mold, utilizing a resin system 61, in which is embedded a plurality of bundles 62, each formed of stacked laminations 64 which have been prebonded together to form coherent bundles of laminations. Each of the bundles 62 of laminations contain a relatively small number of laminations, in order to orient the laminations of each bundle substantially perpendicular to the curved surface of the corner of the opening in the pancake coils. For example, it has been found that bundles formed of one-half inch wide electrical steel laminations having a thickness of 0.12 inch, with the bundles having a build dimension of one-half inch, are very effective in providing a low reluctance magnetic shunt 60. The bundles of laminations may be formed by stacking a large plurality of laminations together and painting their edges with an epoxy resin, or other suitable resin systems, and allowing the resin system to harden. Bundles having the desired number of laminations may then be cut using a sharp tool, from the bonded laminations, and they may then be placed in a mold which is filled with a resin system. The resin system should also be filled with disin FIG. 2 may also be used with the coil support and magnetic shunt member 54.

FIG. 4 is a perspective view of a transformer 70, which illustrates the placement of the specially formed magnetic shunt members, such as shown in FIGS. 2 and 3, adjacent the rounded or curved edges of the openings in the pancake coils, as well as illustrating a new and improved magnetic shielding structure constructed according to the teachings of the invention, which may be utilized with the specially formed corner shield members. Specifically, transformer 70 includes a magnetic core-winding assembly 72 disposed within a tank 84, which may be filled with a suitable insulating and cooling fluid, such as oil. The magnetic core winding assembly 72 includes a plurality of pancake coils 74 disposed in inductive relation with a magnetic core structure 76. Magnetic core structure 76 is formed of first and second magnetic core sections 78 and 80 which are disposed in side-by-side relation to provide a leg portion 82 about which the pancake coils 74 are assembled.

The tank 84 is of the form-fit construction, having a lower portion 83 which closely conforms to the outer configuration of the lower extension of the pancake coils 74 from the magnetic core 76, and which includes a shelf 87 upon which laminations 77 are stacked in superposed relation to form the magnetic core sections 78 and 80. The laminations 77 of the magnetic core sections 78 and 80 are compressed tightly together against shelf 87 by end frame members, such as end frame members 86 and 88, which are welded to the inside of the upper portion 89 of tank 84. The close proximity of the lower tank walls 83 and end frame members 86 and 88 to the pancake coils 74 provides a low reluctance path for the leakage flux from the coils, making it necessary to magnetically shield these components in order to prevent heating and losses therein.

In the prior art, the tank walls and end frame members are shielded with bundles of metallic laminations which are attached to the walls and end frame members, and which form magnetic circuits by butting the ends of the bundles of laminations together. However, the tolerances of the bundles, and the tolerances of the end frame members and tank make it difficult to obtain good butt joints between the ends of the bundles of laminations, with substantial gaps thus being introduced into the magnetic circuit. These gaps increase the reluctance of the circuits and reduce the efficiency of the magnetic shunts.

In addition to shielding the tank walls and end frame memebrs from the leakage magnetic flux from the plurality of pancake coils, in the prior art the coil support members are magnetically shielded, as hereinbefore described relative to FIG. 1. The bundles of metallic laminations are assembled by welding across the edges of the laminations, and then the bundles of laminations themselves are welded to the metallic T-beam or support members. The welds across the edges of the magnetic steel laminations adversely affect their magnetic properties, which increases the reluctance of the magnetic circuits, and it would thus be desirable to be able to assemble the laminations into bundles and fix the location of the bundles, without resorting to welding. FIG. 4 illus-- trates new and improved magnetic shielding assemblies which perform the function of supporting the pancake coils, and of providing low reluctance magnetic circuits between the leg portion of the magnetic coil structure and the pancake coils, and between the pancake coils and the tank and end frame members of the transformer.

More specifically, the new and improved magnetic shielding include lower and upper magnetic shielding assemblies 90 and 92, respectively. The lower magnetic shielding assembly 90 includes a plurality of bundles 94 of laminations 95, which extend in an insulating manner through the pancake coil openings, between the lower surface of the leg portion 82 and the lower end of the openings in the pancake coils 74. The separate metallic support member or T-beam is eliminated by constructing the bundles 94 to have the strength necessary to support the pancake coils 74. The bundles 94 of metallic, magnetic laminations are placed side-by-side across the bottom surface of the pancake coil openings, on the flat portion of the coil opening, with the major planes of the laminations being perpendicular to the adjacent edges of the coil openings. The rounded or curved edges of the coil opening may be filled with specially formed magnetic shunt members 97 and 99, which aid in supporting the laminations of the leg portion of the magnetic core, as well as providing a low reluctance path for the leakage flux which is perpendicular to the curved edges of the coil openings. The specially formed magnetic shunt members 97 and 99 are formed of a cast resin system filled with magnetic metallic material, as hereinbefore described.

The bundles 94 of laminations 95, as well as the specially prepared magnetic shunt members 97 and 99, have their ends at one side of the coils 74 disposed adjacent the end of a plurality of bundles 96 of laminations 101, which bundles are disposed perpendicular to the bundles 94, and which extend towards the bottom 85 of the tank 84. The major planes of the laminations 101 are perpendicular to the insulating barrier 103 disposed at the end of the stack of coils 74. The other ends of the bundles 94, and the other ends of the special magnetic shunts 97 and 99 are disposed adjacent the ends of similarly disposed bundles of laminations (not shown), at the opposite side of the coils 74.

In order to connect the ends of bundles 94 and 96 with a low reluctance joint, the laminations of bundles 94 and 96 may be staggered to provide tongue-and-groove joints, which have a very low reluctance, even when the tongue-and-groove joints are not completely closed. Thus, a liberal tolerance may be utilized while constructing the bundles of laminations and the noncritical portions of the transformer, without substantially affecting the losses of the magnetic circuit.

The tank bottom 85 is shielded from the leakage flux from the pancake coils 74 by a plurality of bundles 98 of laminations 105 which extend between the lower extremity of the pancake coils 74 and the tank bottom 85, and which are magnetically linked with the perpendicularly disposed bundles at each end of the stacked coil assembly, such as the bundles 96. The bundles 98 may be joined to the perpendicularly disposed bundles with tongue-and-groove joints, similar to the connection of the bundles 94 to the perpendicularly disposed bundles at each end thereof. Thus, there is a complete magnetic circuit or loop which encircles the lower portion of the pancake coils 74, which extends through the coil opening to shield the tongue iron from the leakage flux, it extends along the walls of the lower tank portion 83 to shield the adjacent walls from the leakage flux from the pancake coils, and it also extends across the bottom 85 of the tank 84, between the bottom 85 and the pancake coils 74. The lower shielding structure 90 also includes a second complete magnetic loop or circuit which encircles the lower portion of the pancake coils 74 perpendicular to the axis of the first magnetic loop or circuit. This magnetic loop protects the remaining side wall portions of the lower tank portion 83 from leakage flux from the bottom extension of the pancake coils 74 below the magnetic core' structure 76.

More specifically, the second magnetic loop portion of the lower magnetic shielding assembly 90 includes a plurality of bundles 102 of laminations 107 which have their adjacent ends insulatingly butted against the outer lamination of the adjacent outer bundle of the bundles 96 of laminations. In like manner a plurality of bundles 106 are disposed perpendicularly to the bundles 96, with the adjacent ends of the bundles 106 being insulatingly butted against the outer lamination of the bundle at the other outer side of the bundles 96. The bundles #102 are magnetically joined to a plurality of bundles 104, such as by a tongue-and-groove joint, which bundles extend along a lower vertical transformer wall, parallel with the tank bottom 85, between the pancake coils and the adjacent transformer wall, to where their other ends are joined with a plurality of bundles disposed similar to the bundles 102. These bundles then have their ends butted against the bundles at the other end of the transformer which are similar in location to the bundles 96. In other words, bundles 102 and 104 are part of a substantially Us'haped structure 121 which is fitted about one end of the lower extension of the pancake coils 74 below the magnetic core 76, with the legs of the U-shaped structure 121 being butted against the vertically disposed bundles of laminations at opposite ends of the coils '74.

The bundles 106 are magnetically linked with a plurality of bundles 108, such as with tongueand-groove joints, which extend along the adjacent lower vertical transformer wall to protect this .wall from the leakage flux from the pancake coils 74. The bundles 108 join a plurality of bundles disposed similar to bundles 106, except at the other end of the coils, such as with tongueand-groove joints, which bundles extend perpendicularly against the bundles at the other end of the transformer which are disposed similar to bundles 96. Thus, bundles 106 and 108 are part of another U-s'haped structure 123, which, along with the other U-shaped structure 121 and the vertically disposed bundles form a complete magnetic circuit about the sides and ends of the lower extension of the pancake coils 74, which protect the bottom portion 83 of the transformer tank 84 from overheating due to stray leakage flux.

The upper shielding assembly 92 is similar to the lower shielding assembly 90, except that the magnetic circuit which extends through the coil windows is not completed across the top of the coils, since it may interfere with leads and accessories disposed above the pancake coils. Further, it is unnecessary to shield the top of the pancake coils, as the top of the tank 84 is not immediately adjacent the top of the pancake coils. More specifically, the upper shielding assembly 92 includes a plurality of bundles shown generally at 112 which extend through the coil openings, between the upper ends of the openings in the pancake coils 7-4 and the upper surface of the magnetic core structure 76, with the adjacent ends of the bundles 112 at one end of the stacked coil assembly being joined to the ends of bundles 110, which are disposed perpendicularly upward from the bundles 112, and with their remaining adjacent. ends being joined to similarly disposed bundles at the opposite end of the stacked coils, to shield the end frames at opposite ends of the transformer. Two substantially U-shaped structures 125 and 127, similar to the U-shaped structures 121 and 123, are disposed about opposite sides of the upper extensions of the pancake coils 74, with the ends of the U-shaped structures i125 and 1127 butting against the perpendicularly disposed bundles of laminations at opposite ends of the transformer, such as the bundles 110. The first U-shaped structure 125 includes a plurality of bundles 114 which are butted against the perpendicularly disposed bundles 110, a plurality of similarly disposed bundles at the opposite side of the transformer (not shown), and a plurality of bundles I137 which connect the ends of these bundles, such as with tongue-and groove joints.

The second U-shaped structure 127 includes a plurality of bundles 115 disposed with their ends perpendicular to the plurality of bundles 110, a plurality of similarly disposed bundles at the other end of the transformer (not shown), and a plurality of bundles 116 which connect the ends of these bundles, such as with tongue-and-groove joints. Therefore, the upper shielding structure 92 provides a low reluctance path for the leakage flux within the coil openings, and also a low reluctance path for the leakage flux which protects the end frame members from the leakage flux of the pancake coils.

FIG. 5 is a perspective view of bundles 120 and 122 9 of laminations 125, which are constructed according to an embodiment of the invention which enables the bundles to achieve the structural strength required to eliminate the T-beam or coil supports disposed in the upper and lower coil openings between the coil openings and the tongue or leg portion of the magnetic core structure 76. As illustrated in FIG. 5, the bundles 120 and 122 are formed such that they may be joined with tongue-andgroove joints, with either in-line or angular connections between the two bundles. Bundle 120 includes a plurality of laminations 132, 128, and 124 of similar length, separated by a plurality of laminations #12 6 and 130 of a shorter length, in order to provide the tongue-and-groove ends which will mate with the tongue-and-groove ends of bundle 122. In this embodiment the loose laminations are wrapped tightly with a woven fabric tape 131, such as a cotton or glass tape, and the ends of the tape are temporarily secured while the entire tape area is coated and impregnated with a low viscosity resin 133, which is then cured. The low viscosity resin 133, which is preferably a thermosetting resin, penetrates the woven fibers of the fabric tape 131, and penetrates the stacked laminations for a short distance, which, upon curing, bonds the laminations into a coherent bundle having an integrity and strength greater than the bundles of the prior art which are welded across their edges to obtain the desired assembled relation. Instead of impregnating the tape and laminations after assembly, the material of which the laminations are formed, and the tape may be pretreated with resin which is dried but not cured. Then, after the laminations are cut, stacked and taped, the assembly is heated to flow the resin and cure it to a solid.

The adhesive or resin coating 133 may be any suitable resin which will maintain its strength at the elevated temperatures to which it will be subjected during normal usage of the transformer, and it should also be resistant to attack from oils and other dielectric fluids used within the transformer. Epoxy resins have been found to be excellent, because of their strength at elevated temperatures, and their resistance to attack from transformer oils and other chemicals. In addition to forming a solid high strength bundle of laminations, the impregnated and cured resin tape structure provides the insulation required between the shield member and adjacent portions of the transformer.

While the bundles of laminations for sielding the end frames and tank of the transformer have been illustrated as being assembled and mounted on the pancake coils 74, it will also be evident that these magnetic shielding structures may be connected to the end frames and tank Walls by suitable clips which are sized to hold the bundles and which are welded to the end frames and tank walls.

In summary there has been disclosed a new and improved magnetic shielding arrangement in which separate coil supports for the pancake coils of a shell-form transformer have been completely eliminated. The coil supports are provided by bundles of metallic laminations which are assembled by wrapping a tape tightly about the laminations, impregnating the tape with a suitable resin system, and curing the resin to form a coherent solid. Other similarly prepared bundles of laminations are disposed to form complete magnetic circuits about the upper and lower extensions of the pancake coils, as well as a complete magnetic circuit through the coil opening, adjacent to the lower tank walls, and adjacent to the tank bottom. Most of the joints between the ends of he bundles of laminations which form the magnetic circuits are of the tongue-and-groove type, which allow a substantial tolerance in the manufacturing of the bundles, as well as in the tank walls and end frames which they are to shield, without substantially impairing the magnetic efficiency of the joints.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made Without departing from the spirit thereof, it is intended that all matter contained in the foregoing descripion or shown in the accompanying drawings shall be interpreted as illustrative, and not in a limiting sense.

We claim as our invention:

1. An electrical transformer comprising:

a magnetic core structure having at least two spaced windows which extend between predetermined first and second opposite surfaces of said magnetic core structure to define at least one leg portion,

a plurality of electrical coils assembled on said leg portion, with said electrical coils extending outwardly from said predetermined first and second opposite surfaces of said magnetic core structure,

first and second magnetic means disposed between said leg portion and said electrical coils at said predetermined first and second opposite surfaces of said magnetic core structure, respectively, said first and second magnetic means including a plurality of stacked metallic laminations held together with insulating means to provide bundles of laminations disposed such that each lamination extends between said leg portion and said electrical coils, to support said electrical coils relative to said magnetic core structure, and also provide a low reluctance path for leakage magnetic flux,

a plurality of additional bundles of metallic laminations, certain of said additional bundles of laminations being disposed perpendicular to the plane of the first surface of the magnetic core structure and joining the ends of the first magnetic means with tongue and groove joints, and certain of said additional bundles of laminations being disposed perpendicular to the plane of the second surface of the magnetic core structure and joining the ends of the second magnetic means with tongue and groove joints.

2. The electrical transformer of claim 1 wherein the integrity of each bundle of metallic laminations is at least partially maintained by a fabric wrapped about the laminations, which is impregnated with a cured resinous insulating means.

3. The electrical transformer of claim 1 wherein certain of the additional bundles of metallic laminations are disposed adjacent the portion of' the electrical coils which extends outwardly from the first surface of the magnetic core structure, the ends of which magnetically link the free ends of the bundles of laminations disposed perpendicular to the first surface of the magnetic core structure, to provide a closed magnetic circuit about the portion of the electrical coils which extend outwardly from the first surface of the magnetic core structure.

4. The electrical transformer of claim 3 wherein all of the magnetically linked bundles of laminations are mechanically joined with tongue-and-groove joints.

5. The electrical transformer of claim 1 wherein certain of the bundles of metallic laminations are arranged to provide a plurality of substantially U-shaped structures which are disposed about the portions of the electrical coils which extend outwardly from the first and second surfaces of the magnetic core structure, such that the ends of the legs of the substantially U-shaped structure butt against certain of the bundles of laminations disposed perpendicular to the first and second surfaces of the magnetic core structure, to provide closed magnetic loops about the portion of the electrical coils which extend outwardly from the first and second surfaces of the magnetic core structure.

6. The electrical transformer of claim 5 wherein the plurality of substantially U-shaped structures are formed by assembling the bundles with tongue-and-groove joints.

7. The electrical transformer of claim 5 wherein certain of the bundles of metallic laminations are disposed adjacent the extreme surfaces of the electrical coils which extend outwardly from the first surface of the magnetic core structure, the ends of which magnetically link the free ends of the bundles of laminations disposed perpendicular to the first surface of the magnetic core structure, to provide an additional closed magnetic circuit about the portion of the electrical coils which extend outwardly from the first surface of the magnetic core structure.

=8. The electrical transformer of claim 7 wherein the plurality of substantially U-shaped structures are formed by assembling the bundles with tongue-and-groove joints, and wherein the additional closed magnetic circuit is formed by mechanically connecting bundles of laminations with tongue-and-groove joints.

9. The electrical transformer of claim 1 wherein the bundles of metallic laminations of the first and second magnetic means, and the plurality of additional bundles of laminations each have their integrity maintained by a fabric tape wrapped about the laminations, which is impregnated with a cured resinous insulating means.

References Cited UNITED STATES PATENTS THOMAS J. KOZMA, Primary Examiner US. Cl. X.R. 336212, 216, 219 

